Use constness query for const_trait, too

compiler/rustc_const_eval/src/const_eval/fn_queries.rs

@@ -3,8 +3,9 @@ use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
+use rustc_span::sym;
 
-pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
+fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     let parent_id = tcx.local_parent(def_id);
     matches!(tcx.def_kind(parent_id), DefKind::Impl { .. })
         && tcx.constness(parent_id) == hir::Constness::Const
@@ -25,6 +26,14 @@ fn constness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Constness {
             hir::Constness::Const
         }
         hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.constness,
+        hir::Node::Item(hir::Item { kind: hir::ItemKind::Trait(..), .. }) => {
+            if tcx.has_attr(def_id, sym::const_trait) {
+                hir::Constness::Const
+            } else {
+                hir::Constness::NotConst
+            }
+        }
+
         hir::Node::ForeignItem(_) => {
             // Foreign items cannot be evaluated at compile-time.
             hir::Constness::NotConst

compiler/rustc_hir_analysis/src/collect.rs

@@ -1135,13 +1135,6 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
         _ => span_bug!(item.span, "trait_def_of_item invoked on non-trait"),
     };
 
-    // Only regular traits can be const.
-    let constness = if !is_alias && tcx.has_attr(def_id, sym::const_trait) {
-        hir::Constness::Const
-    } else {
-        hir::Constness::NotConst
-    };
-
     let paren_sugar = tcx.has_attr(def_id, sym::rustc_paren_sugar);
     if paren_sugar && !tcx.features().unboxed_closures() {
         tcx.dcx().emit_err(errors::ParenSugarAttribute { span: item.span });
@@ -1299,7 +1292,6 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
     ty::TraitDef {
         def_id: def_id.to_def_id(),
         safety,
-        constness,
         paren_sugar,
         has_auto_impl: is_auto,
         is_marker,

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1269,6 +1269,7 @@ fn should_encode_constness(def_kind: DefKind) -> bool {
         | DefKind::Closure
         | DefKind::Impl { of_trait: true }
         | DefKind::Variant
+        | DefKind::Trait
         | DefKind::Ctor(..) => true,
 
         DefKind::Struct
@@ -1287,7 +1288,6 @@ fn should_encode_constness(def_kind: DefKind) -> bool {
         | DefKind::InlineConst
         | DefKind::AssocTy
         | DefKind::TyParam
-        | DefKind::Trait
         | DefKind::TraitAlias
         | DefKind::Mod
         | DefKind::ForeignMod

compiler/rustc_middle/src/ty/mod.rs

@@ -2067,7 +2067,8 @@ impl<'tcx> TyCtxt<'tcx> {
 
     #[inline]
     pub fn is_const_trait(self, def_id: DefId) -> bool {
-        self.trait_def(def_id).constness == hir::Constness::Const
+        debug_assert_eq!(self.def_kind(def_id), DefKind::Trait);
+        self.constness(def_id) == hir::Constness::Const
     }
 
     #[inline]

compiler/rustc_middle/src/ty/trait_def.rs

@@ -20,9 +20,6 @@ pub struct TraitDef {
 
     pub safety: hir::Safety,
 
-    /// Whether this trait has been annotated with `#[const_trait]`.
-    pub constness: hir::Constness,
-
     /// If `true`, then this trait had the `#[rustc_paren_sugar]`
     /// attribute, indicating that it should be used with `Foo()`
     /// sugar. This is a temporary thing -- eventually any trait will